It is known that semiconductor memory devices, such as for example non-volatile memories of the ROM, EPROM, EEPROM and Flash EEPROM families, comprise a matrix of memory cells arranged in rows ("word lines") and columns ("bit lines"), and selection circuits of the rows and columns. For reading the memory sensing circuit are provided ("sense amplifiers") adapted to detect the programming state of those memory cells belonging to the selected rows and columns.
The read operation normally provides an initialization phase during which the potentials of the selected rows and columns reach prescribed initial levels, and the sensing circuits are initialized.
Only after said phase the evaluation phase of the data stored in the selected memory cells starts.
Typically, in the initialization phase the potential of the selected rows is brought to a prescribed value, e.g. that of the supply voltage of the integrated circuit; at the same time, the potential of the selected columns is brought to an initial prescribed level too. In the case of the sense amplifier having the known comparison architecture, providing for comparing the current sunk by the addressed memory cells with reference memory cells, the potentials of the columns of memory cells, the columns of reference memory cells and the internal potentials of the sensing circuits are equalized.
In the following phase of evaluation of the data, the equalization of the potentials of the matrix columns and the reference columns and of the internal nodes of the sensing circuits ceases, and such potentials evolve according to the programming state of the addressed cells.
In order to guarantee the correct working of the memory, it is important to accurately determine the duration of the initialization phase, i.e. the time instant at which the initialization phase ends and the data evaluation phase starts.
A problem in the design phase is that the optimum time duration of the initialization phase cannot be considered an absolute constant, being actually a function of the parameters of the memory cells, of the supply voltage, of the working temperature, and other variables, such as the sensibility of the sense amplifiers.
For example, in a memory device wherein the memory cells are characterized by a conductivity rather low it is necessary that the initialization phase has a longer duration. The same holds true in the case that the memory device is operated at a low supply voltage; when the supply voltage is so low that a reliable reading of the data cannot be assured, the data evaluation phase should not even be started.
The choice of a duration too small of the initialization phase thus has the consequence that, in certain situations, the data provided at the output of the memory are not correct.
However, choosing an initialization phase with too long a duration could cause that the prescription regarding the memory access time is not complied.
In other words, an initialization phase with a constant duration, independently of the working conditions of the memory and other conditions, poses problems.
The known timing circuits used to generate time delays allow in general to have time delays which are not correlated to the electric characteristics of the memory cells to be read and their operating conditions.